This invention generally relates to methods and apparatus for ultrasonic imaging of blood flow parameters.
The most common modes of diagnostic ultrasound imaging include B- and M-modes (used to image internal, physical structure), spectral Doppler, and color flow (the latter two primarily used to image flow characteristics, such as in blood vessels). The color flow mode is typically used to detect the velocity of blood flow toward/away from the transducer, and it essentially utilizes the same technique as is used in the spectral Doppler mode. Whereas the spectral Doppler mode displays velocity versus time for a single selected sample volume, color flow mode displays hundreds of adjacent sample volumes simultaneously, all laid over a B-mode image and color-coded to represent each sample volume's velocity.
Measurement of blood flow in the heart and vessels using the Doppler effect is well known. The phase shift of backscattered ultrasound waves may be used to measure the velocity of the backscatterers from tissue or blood. The Doppler shift may be displayed using different colors to represent speed and direction of flow. Alternatively, in power Doppler imaging, the power contained in the returned Doppler signal is displayed.
Conventional ultrasound imaging systems comprise an array of ultrasonic transducer elements arranged in one or more rows and driven with separate voltages. By selecting the time delay (or phase) and amplitude of the applied voltages, the individual transducer elements in a given row can be controlled to produce ultrasonic waves which combine to form a net ultrasonic wave that travels along a preferred vector direction and is focused at a selected point along the beam. The beamforming parameters of each of the firings may be varied to provide a change in maximum focus or otherwise change the content of the received data for each firing, e.g., by transmitting successive beams along the same scan line with the focal point of each beam being shifted relative to the focal point of the previous beam. In the case of a steered array, by changing the time delays and amplitudes of the applied voltages, the beam with its focal point can be moved in a plane to scan the object. In the case of a linear array, a focused beam directed normal to the array is scanned across the object by translating the aperture across the array from one firing to the next.
The same principles apply when the transducer probe is employed to receive the reflected sound in a receive mode. The voltages produced at the receiving transducer elements are summed so that the net signal is indicative of the ultrasound reflected from a single focal point in the object. As with the transmission mode, this focused reception of the ultrasonic energy is achieved by imparting separate time delays (and/or phase shifts) and gains to the signal from each receiving transducer element.
A single scan line (or small localized group of scan lines) is acquired by transmitting focused ultrasound energy at a point, and then receiving the reflected energy over time. The focused transmit energy is referred to as a transmit beam. During the time after transmit, one or more receive beamformers coherently sum the energy received by each channel, with dynamically changing phase rotation or delays, to produce peak sensitivity along the desired scan lines at ranges proportional to the elapsed time. The resulting focused sensitivity pattern is referred to as a receive beam. A scan line's resolution is a result of the directivity of the associated transmit and receive beam pair.
A B-mode ultrasound image is composed of multiple image scan lines. The brightness of a pixel is based on the intensity of the echo return from the biological tissue being scanned. The outputs of the receive beamformer channels are coherently summed to form a respective pixel intensity value for each sample volume in the object region or volume of interest. These pixel intensity values are log-compressed, scan-converted and then displayed as a B-mode image of the anatomy being scanned.
In addition, ultrasonic scanners for detecting blood flow based on the Doppler effect are well known. Such systems operate by actuating an ultrasonic transducer array to transmit ultrasonic waves into the object and receiving ultrasonic echoes backscattered from the object. The sequence of transmitting waves and receiving echo signals is repeated several times for the same scan line and focal positions. The set of echo signals resulting from identical acquisitions is referred to as an ensemble. Since the ensemble is comprised of beams with identical beamforming the only difference among the beams is the information about the position of the scatterers. Position changes of the scatterers translate into phase shifts in the received signals. The phase shifts further translate into the velocity of the blood flow. The blood velocity is calculated by measuring the phase shift from firing to firing at a specific range gate.
Color flow images are produced by superimposing a color image of the velocity of moving material, such as blood, over a black and white anatomical B-mode image. Typically, color flow mode displays hundreds of adjacent sample volumes simultaneously laid over a B-mode image, each sample volume being color-coded to represent velocity of the moving material inside that sample volume at the time of interrogation.
In state-of-the-art ultrasonic scanners, the pulsed or continuous wave Doppler waveform is also computed and displayed in real-time as a gray-scale spectrogram of velocity versus time with the gray-scale intensity (or color) modulated by the spectral power. The data for each spectral line comprises a multiplicity of frequency data bins for different frequency intervals, the spectral power data in each bin for a respective spectral line being displayed in a respective pixel of a respective column of pixels on the display monitor. Each spectral line represents an instantaneous measurement of blood flow.
The color flow and spectral Doppler techniques have their respective advantages and shortcomings. The color flow Doppler technique displays the mean and possibly the variance of the local blood flow velocity from a region of interest. The velocity information is color coded and overlaid onto an anatomical grayscale image. Flow abnormalities can be observed easily due to the two-dimensional display of the flow distribution. The shortcoming of color flow Doppler imaging is that only the mean velocity and variance can be determined. More advanced parameters such as peak velocity, resistance index, and pulsatility index cannot be assessed using color flow Doppler imaging. Further, the time resolution of color flow Doppler imaging is limited by its frame rate. Typical values are in the range of 50 to 100 msec. In comparison, tracking the systolic flow in the beating heart requires a time resolution of 10 to 20 msec.
In low-flow situations the color flow frame rate can be increased by interleaving pulses in two or more directions. In this scheme two or more color flow ensembles are formed simultaneously by sending consecutive pulses into the ensemble directions before repeating the sequence and sending another pulse into the first direction.
Spectral Doppler ultrasound imaging acquires flow information from a single location at a much higher rate and for a longer period of time. Pulses are transmitted and received at the PRF (pulse repetition frequency), which can be 1000 times higher than the frame rate in color flow imaging. Further the acquisition of spectral data for a given location is continuous, i.e., not disrupted by the acquisition of data in the many other locations needed to form a 2-D image. The continuous acquisition spectral Doppler samples allows one to calculate the entire Doppler spectrum (as opposed to only the mean and variance in color flow imaging). Diagnostic information is extracted from the shape of the spectrum as well as spectral changes during the cardiac cycle. For example, the peak velocity can be determined as a function of time, yielding critical information for the staging of vascular stenoses or cardiac valve regurgitations. An example of the evaluation of spectral changes is the determination of the “resistance index” and the “pulsatility index”. These indices are determined by relating the peak velocities at two different times within the cardiac cycle (end systole and end diastole). Current color flow imaging techniques cannot determine such indices because the time resolution is too poor and the exact spectral shape is unknown. The disadvantage of spectral Doppler imaging, however, is that the flow parameters are determined at only a single spatial location. The spatial distribution of the flow parameters is not easily accessible. In fact, it can only be determined by repeatedly performing the spectral measurements at different locations. However, such an approach is inaccurate, time consuming and expensive, i.e., undesirable.
There is a need for a method that will allow spectral Doppler processing to be performed at every color flow range gate location to create a two-dimensional image.